1. Field of the Invention
This invention relates to a novel polyaluminum dialkyl phosphate, an oil gelling agent comprising such a polyaluminum dialkyl phosphate, and a composition for external application comprising such a polyaluminum dialkyl phosphate which utilizes the characteristic features of such a such a polyaluminum dialkyl phosphate.
2. Description of the Background
Metallic soaps, fatty acid dextrin esters, metallic salts of dialkyl phosphate, organic-denatured bentonites, or the like have been used, because of their gelling capabilities as a gelling agent for medicines and cosmetics an water-in-oil (w/o) type emulsion stabilizer, a pigment dispersing agent for cosmetics and paint, paint flow-prventives, and the like.
These metallic soaps, fatty acid dextrin esters, and metallic salts of dialkyl phosphate, however, when heated and dissolved into an oil and then cooled to a temperature below the phase inversion point (Tc) of the system, cause the system to become a brittle, solid gel. If a stress is applied to this gel, the material is deformed and can not restore its original shape. It must be heated to a temperature above Tc in order to restore the original shape. Because of this, those compounds having a branched, long alkyl group in their molecule and a Tc below a room temperature are used. These compounds, however, produce a gel which is too viscous. Such a gel is cobwebby (draws filaments), providing inconvenience and unacceptable feeing to the users.
Organic-denatured bentonites have, therefore, been used as a gelling agent for providing thixotropic rheological characteristics to oils. Organic-denatured bentonites, however, are produced through intercalation of an alkyl amine on montmorillonite which is a clay mineral. This poses a problem in view of safety of alkyl amine when the material is used for medicines or cosmetics. Furthermore, the turbidity caused by an organic-denatured bentonite because of inclusion of montmorillonite sometimes affects the color of the composition to which this substance is formulated. For this reason the use of organic-denatured bentonites for compositions requiring a delicate color tone is not suitable. In addition, this gelling agent is dispersed in an oil with the aide, as a carrier, of a smectite layer which is a layer constituting montmorillonite and has an expansion of 0.1 to several microns. This tends to leave a large proportion of oil unwrapped and free, when the carrier disperses into oil and wraps the oil, preventing a dense and soft gel from being produced. Because of this the gelling agent only produces rough and unsoft gels.
There is, therefore, a strong need for the development of a gelling agent which can provide thixotropic rheological characteristics to oils, possesses a high degree of safety, is transparent, gives a good feeling to the users, and is free from cobweb (does not draw filaments).
In view of this situation, the present inventors have undertaken extensive studies, and as a result have found that a novel polyaluminum dialkyl phosphate can provide a gelling agent satisfying the above requirements.